The present invention relates to a millimeter-wave antenna system in which the influence of sunlight collected near a focal point of an antenna is reduced, and relates to a method for manufacturing such an antenna system.
As such a millimeter-wave antenna system, hitherto there is an antenna system which has a configuration shown in FIG. 10. FIG 10 is a schematic view of such a background-art antenna system. In FIG. 10, the reference numeral 1 represents an antenna panel; 2, a coating film painted in white or with a semi-gloss on the front surface of the antenna panel 1; and 3, an assistant reflecting mirror disposed on the coating film 2 side. Incident millimeter waves are focused on the assistant reflecting mirror 3 by the antenna panel 1. On the other hand, incident light which is shorter in wavelength than the millimeter waves (not longer than 1 xcexcm) is not focused on the assistant reflecting mirror 3 because the incident light is irregularly reflected by the front surface of the coating film 2. However, if the frequency used in such an antenna system is higher, there is a problem that the millimeter waves passing through the inside of the coating film 2 are attenuated so that the antenna sensitivity is lowered or the observed signals to noise ratio deteriorates.
In order to solve such a problem, in the background art, for example, there is an antenna system disclosed in JP-A-62-131611. FIG. 11 is a schematic sectional view of the background-art millimeter-wave antenna system. In FIG. 11, fine irregularities 4 are formed on one of the surfaces of an antenna panel 1. In this background-art antenna system, the antenna panel 1 which has a diameter of about 50 cm and which can be used without painting is manufactured in the following method. That is, very small grooves are formed in the front surface of a flat plate, and the flat plate is bent and deformed to form the antenna panel 1. Thus, the coating film 2 is eliminated so that the antenna panel 1 can be used even in a high frequency. Further, the fine irregularities 4 are provided on the front surface of the antenna panel 1 so as to restrain light, which is reflected by the antenna panel 1, from being collected on a light collecting portion. Thus, the temperature of the light collecting portion is prevented from rising.
Thus, the aforementioned fine irregularities 4 in the background-art antenna system are formed by scratching, blasting or etching the front surface of the antenna panel 1. Aluminum plates which were 2 mm thick and which were polished on their front surfaces were used for measuring their light reflecting properties. FIG. 12 is an explanatory view for explaining the measuring method.
In the measuring method, as shown in FIG. 12, a flat-plate sample was made just opposite to the sun, and measured with an illuminometer provided with a cylinder so as to detect light only from the front, while the angle of the illuminometer with respect to the flat plate sample was changed. FIGS. 13 and 14 are graphs of the reflecting properties, respectively, of a #40 polished product and a #320 polished product. The RMS (Root Mean Square) front-surface roughness of the #40 polished product was 4.4 xcexcm, and that of the #320 polished product was 0.7 xcexcm. Incidentally, #40 and #320 designate the granularities (sizes) of abrasive grains respectively. The abrasive grain size becomes smaller as the number is larger. FIGS. 13 and 14 show that the illuminance was very high near the angle of 0xc2x0 in each product, and light was not scattered so much. However, because it is assumed in the background-art antenna system that the diameter of the antenna panel is about 50 cm, a little effect can be generated even by such reflecting properties.
Theoretically, to what extent the reflecting properties are required is determined as follows. FIG. 15 is a relationship graph showing the relationship between the ratio of scattered light collected on an assistant reflecting mirror (450 mm) which is in a focus portion and the scatter angle (half width.), in the case of a large-size millimeter-wave parabolic antenna system which is of the order of 10 m. In this case, the energy entering a parabolic antenna is 1xc3x97105 W when the parabolic antenna is just opposite to the sun. When this energy is multiplied by the light collecting ratio and the absorptivity of the assistant reflecting mirror, the temperature rises by about 100xc2x0 C. if the light collecting ratio is 0.015. Such temperature rising is considered to be the limit in use if the time for the parabolic antenna to be just opposite to the sun is not so long. On the other hand, the scattering angle is 55 degrees when the light collecting ratio is 0.015. Accordingly, the scattering angle has to be larger than the above value 55 degrees. The scattering angle was 10 degrees in the aforementioned samples which were mechanically polished. According to similar calculation, the temperature rises up to 2,000xc2x0 C. when the light collecting ratio is 0.3. Since this temperature exceeds the melting point of aluminum (Al), the samples are not applicable.
In addition, in order to use also in high frequency, the RMS front-surface roughness has to be not larger than 10 xcexcm all over the antenna panel which is of the order of 10 m. Therefore, when the antenna panel is finished, the warp (distortion) of the antenna panel has to be suppressed as small as possible. Even if a good mirror surface accuracy of the antenna panel can be obtained by machining before polishing, there may be a case that a predetermined mirror surface accuracy cannot be obtained after the front surface of the antenna panel is polished. Further, when a very high-precision antenna panel aimed at millimeter waves and submillimeter waves is manufactured and in order to finish the front surface of the antenna panel into a mirror surface with less xe2x80x9cswellxe2x80x9d by machining or the like, it is difficult to groove the mirror surface beforehand. In addition, it is very difficult to polish the curved surface of the antenna panel after the antenna panel is formed to have a mirror surface.
Therefore, the present invention was developed to solve the foregoing problems. It is an object of the present invention to provide a novel antenna system in which fine irregularities are formed on the front and back surfaces of an antenna panel by blasting so that the warp of the antenna panel can be reduced to ensure the front surface accuracy of the antenna panel while sunlight can be restrained from being collected; and to provide a method for manufacturing such an antenna system.
According to an aspect of the present invention, as stated in Aspect 1, an antenna system comprises an antenna panel which has fine irregularities on front and back surfaces thereof, the fine irregularities being formed by blasting so as to regularly reflect service radio waves having longer wavelength than sunlight and scatter sunlight.
Preferably, as stated in Aspect 2, in the antenna system according to Aspect 1, a chemical surface film is formed on the front surface of the antenna panel where the fine irregularities have been formed.
Preferably, as stated in Aspect 3, in the antenna system according to Aspect 1 or 2, the fine irregularities are in a range of from 0.1 xcexcm to 1.0 xcexcm in RMS front-surface roughness.
Preferably, as stated in Aspect 4, in the antenna system according to Aspect 2 or 3, the chemical surface film is formed by alodine processing.
Preferably, as stated in Aspect 5, in the antenna system according to Aspect 1 to 4, the chemical surface film is colorless.
According to another aspect of the present invention, as stated in Aspect 6, a method for manufacturing an antenna system in which an antenna panel is manufactured by combining a plurality of constituent parts, comprises the steps of: finishing front and back surface portions of each of the constituent parts so as to thin down the constituent parts; blasting the front and back surface portions of the constituent parts with abrasive grains after the finishing step so as to form fine irregularities on front and back surfaces of the antenna panel; combining the plurality of constituent parts subjected to the blasting step so as to form the antenna panel into a desired shape; and attaching an assistant reflecting mirror to the antenna panel acting as a main reflecting mirror.
Preferably, as stated in Aspect 7, the method according to Aspect 6 further comprises the step of applying surface treatment to the constituent parts subjected to the blasting step.
Preferably, as stated in Aspect 8, in the method according to Aspect 7, the surface treatment step is performed by application of alkali washing or acid washing.